Section nine of chapter 40A authorizes zoning ordinances or by-laws to provide for special permits for uses that are in harmony with the general purpose and intent of the ordinance or bylaw, including special permits for cluster developments. Such cluster developments are to be permitted upon review and approval by a planning board pursuant to the subdivision control laws of chapter forty-one, sections 81K to 81GG inclusive. See paragraph six. Paragraph seven defines cluster development, as among other things, residential developments separated by intervening open lands. This paragraph also describes proper management of these open lands.